1. Field of the Invention
The present invention relates to a pneumatic tire. More particularly, the present invention relates to a pneumatic tire whose wet performance and tire noise performance are not reduced or improve when the tire becomes worn.
2. Background Information
Conventionally, there is a known pneumatic tire having a tread pattern including a plurality of straight grooves extending in a tire circumferential direction in order to improve wet performance such as hydroplaning performance during traveling on a wet road. Such a tire can effectively drain out water on the road surface to a rear side of the tire and suppress entry of the water into a tire contact patch to thereby maintain satisfactory wet performance such as hydroplaning performance.
However, during traveling on a dry road, groove widths and groove areas of the straight grooves extending in the circumferential direction rapidly shrink and expand to initial states in the contact patch due to a tire deformation to cause vibration in side walls of the grooves. This vibration makes air vibrate in the circumferential grooves with air cavity resonance, and increases tire noise. Japanese Patent Application Laid-Open No. 7-40712 describes a tire having a tread pattern in which groove widths of circumferential grooves are increased and decreased in a tire width direction to impart operations of silencers to the grooves to cope with this problem.
In the tire described Japanese Patent Application Laid-Open No. 7-40712, when a distance from one lug groove to the adjacent lug groove in the circumferential direction is defined as one pitch which is a minimum unit for forming the pattern of a tread portion, each of the circumferential grooves has a plurality of wide portions having increased groove widths and a plurality of narrow portions having smaller widths within a pitch, and distances between the wide portions or distances between the narrow portions in the circumferential direction are different from each other. With this tire, the noise can be reduced while maintaining wet performance and especially hydroplaning performance as compared with a related tire.